A sensor element selected to provide an output signal in response to variation in a parameter will typically be responsive to other parameters giving rise to unwanted signal variations. Such unwanted variations may be compensated by providing an element sensitive predominantly to the unwanted parameter and using its output signal to subtract the unwanted effect from the sensor output so that such variations are calibrated out. The calibration procedure of many sensor types includes providing such an element.
Calibration is also normally required to provide compensation between nominally identical sensors, since typically there will be differences in the unwanted variations between sensors of the same type due to manufacturing tolerance, differences in batch materials, etc. Silicon piezo-resistor pressure sensors (PRTS), for example, exhibit a wanted pressure characteristic varying with temperature. This unwanted variation is in addition to other intrinsic features requiring calibration such as offset variation and gain variation between sensors.
A typical PRT is a four terminal device providing a differential output voltage between two terminals which increases with applied pressure, the other terminals serving as power supply connections. For a description of such a device reference may be made to U.S. Pat. No. 4,317,126 issued to Gragg. There are three principle characteristics of the device to be calibrated, offset, that is a non-zero output for the reference input magnitude, gain, that is variations in the magnitude of the output voltage between sensors at a constant input, and temperature, which manifests itself as a temperature dependent offset to be compensated by a temperature sensitive device such as a thermistor.
In PRT technology it is known to implement a calibration network in a hybrid circuit packaged together with the sensor to produce transducer components of repeatable performance.